The present invention relates to equipment for diagnostic and therapeutic radiology and methods of making the same and, more particularly, to an emissive coating for x-ray tube rotors, such as those utilized in x-ray tubes.
One problem faced by x-ray tube designers has been related to the amount of heat generated during the x-ray generation cycle. Specifically, the silver lubricated bearings used with the anode rotor have, in the past, had a tendency to fail prematurely due to overheating from the tremendously high temperatures generated in the x-ray tube during peak power situations. Specifically, it is not uncommon for temperatures in the range of 700.degree. C. to be generated in the vicinity of the silver lubricated bearing most proximate the rotating target. The problem related to rotor bearing overheating had been effectively solved utilizing an emissive coating on the anode rotor by plasma spraying a 0.001 inch thick oxygen deficient TiO.sub.2 coating onto the rotor skirt.
With the recent tendency toward higher and higher power x-ray tubes and for nearly continuous twenty-four hour, seven day a week operations, another problem has developed with the anode rotor, that being material flaking from the surface of the rotor. This flaking or spallation of the brittle TiO.sub.2 created fine particles that migrated to high electrical field regions of the tube, thus causing high voltage instabilities and arcing.
Recent GE autopsy results on certain x-ray tubes indicate that flaking of the rotor emissive coating is associated with about 62% of tube failures in the field. Thermal expansion studies of the plasma sprayed TiO.sub.2 emissive coating used indicate that it begins sintering at temperatures as low as 350.degree. C. and appears to shrink as much as 0.2% when heated to 800.degree. C. It is believed that the brittle TiO.sub.2 stress-relieves by sintering when it is exposed to its maximum operating temperature of about 700.degree. C., and then fails compressively during thermal cycling.
Recently, the problem related to rotor flaking had reached a critical point. Due to the tremendous load stresses undergone by certain x-ray tubes during continuous operation, the average tubes life had been approximately 28,000 scan-seconds, utilizing the old TiO.sub.2 rotor coating. Since an approximate 28,000 scan-second life did not even approach the 50,000 scan-second life per x-ray tube warranty and approximately 60% of the failures were due to flaking of the anode rotor, the need for an improved rotor having a coating that would eliminate the flaking while maintaining the effectiveness of the thermal emissive properties became apparent. Such a rotor coating composition desirably would provide sufficient thermal protection for the bearings and have sufficient emissive characteristics, while reducing significantly, if not eliminating, entirely flaking of the rotor coating such that the average x-ray tube life would more closely approach the guaranteed 50,000 scan-seconds life warranty.